The present invention relates generally to direct processing of metal passing through an electroslag refining operation. More specifically, it relates to an apparatus and method having a split, insulated crucible that provides current through a non-consumable electrode with at least one pair of symmetrical electrical leads in the electroslag processing apparatus. The invention further relates to atomizing, or otherwise directly processing a stream of refined metal, which stream is generated directly beneath an electroslag processing apparatus.
It is known that the processing and refining of relatively large bodies of metal, such as superalloys, are accompanied by many problems due to the bulky volume of the body of metal itself. One such problem is controlling the grain size and other microstructure of the refined metals. Because the volume of the metal being refined is generally in the order of about 5,000 to about 35,000 pounds or more, the refining processing involves multiple steps, such as sequential heating and melting, forming, cooling, and reheating of the large bodies of metal. Further, as processing by melting and similar operations is carried out on large bodies of metal, problems of segregation of the alloy elements or ingredients of the metal also occur. Often, a lengthy and expensive sequence of processing operations is selected in order to overcome the above-mentioned difficulties which arise through the use of bulk processing and refining operations of metals.
One such sequence of steps used in industry, involves vacuum induction melting; followed by electroslag refining; followed, in turn, by vacuum arc refining and followed, again in turn, by mechanical working through forging and drawing. While the metal produced by such a sequence is highly useful and the metal product itself is quite valuable, the processing sequence is quite expensive and time-consuming.
For example, vacuum induction melting of scrap metal into a large body of metal, such as between 20,000 to about 35,000 pounds or more, can be very useful for the recovery of the scrap material. The scrap and other metal is processed through the vacuum induction melting steps to form a large ingot. Such a formed ingot has considerably more value than the scrap and other material used to form the ingot. However, in accordance with the conventional vacuum induction melting process, the large ingot product is usually found to contain one or more of three types of defects: specially voids, slag inclusions, and macrosegregation.
The recovery of scrap metal into an ingot is usually the first step in an expensive, time-consuming metal refining process. Some of subsequent processing steps are specifically to cure the defects generated during prior metal processing steps. For instance, after the scrap metal is formed into a large ingot, it then is often processed through an electroslag refining step to remove oxides and sulfides. The product of the electroslag refining process containes lower concentrations of these impurities.
However, problems also occur during the conventional electroslag refining process. Briefly, the conventional electroslag process includes a refining vessel containing a slag refining layer floating on a layer of molten refined metal. An ingot of unrefined metal is used as a consumable electrode and is lowered into the vessel to make contact with the molten electroslag layer. A refining current is passed through the slag layer to the ingot and causes surface melting at the interface between the ingot and the slag layer. As the ingot is melted, oxide inclusions or impurities are exposed to the slag and removed from the metal at the point of contact between the ingot and the slag. Droplets of refined metal are formed and these droplets pass down through the slag to be collected in a pool of molten refined metal beneath the slag.
The apparatus mentioned above, having an ingot as a consumable electrode, includes a fixed relationship between the individual parameters of the process and, in particular, between the intensity of the refined current, the specific heat input, and the melting rate. This fixed relationship entails undesirable interdependence between the rate of electroslag refining of the metal, the metal ingot temperature and the rate at which the refined molten metal is cooled. In addition, there are problems concerning preparation of a large consumable electrode ingot. Further, in the past, it has been difficult for a conventional electroslag process utilizing a consumable electrode to provide active stirring of the metal and the slag. Thus, it would be desirable to provide an apparatus that does not need to use a consumable electrode ingot. It is also desirable to provide an apparatus that increases the active stirring of the metal and the slag to essentially improve the refining effect of the electroslag process.
Another problem of conventional electroslag refining is the formation of a relatively deep metal pool in the electroslag crucible. This deep melt pool causes a varied degree of ingredient macrosegregation which leads to a less desirable microstructure in the end product. To overcome this deep melt pool problem, a subsequent processing operation is employed in combination with the electroslag refining process. This latter processing may typically be vacuum arc refining. Vacuum arc refining is initiated when the ingot produced by electroslag refining is processed through the vacuum arc steps to produce a relatively shallow melt pool whereby an improved microstructure, perhaps also having a lower hydrogen content, is produced. Following the vacuum arc refining process, the resulting ingot is then mechanically worked to yield a metal stock having a better microstructure. Such mechanical working may involve a combination of steps of forging and drawing. This thermo-mechanical processing requires large, expensive equipment, as well as costly amounts of energy input.
As pointed out, the drawbacks to using the above-recited combination of process steps are many. Thus there is a need for a simplified, less costly, and time efficient method and apparatus for processing metals.
A method and apparatus which permit formation of relatively large ingots of metal of uniform composition and desirably fine microstructure without the need for extensive processing has been previously suggested by the General Electric Company in a number of patents (U.S. Pat. Nos. 5,160,532; 5,310,165; 5,325,906; 5,332,197; 5,348,566 and 5,366,206).
The methods described in these patents involve a refining vessel containing an electroslag refining layer floating on a layer of molten refined metal with a consumable electrode ingot of unrefined metal. The droplets of the refined metal that are formed pass through the slag and are collected in a pool of molten refined metal beneath the slag. This refined metal is held in a cold hearth. At the bottom of the cold hearth, a cold finger orifice permits the withdrawal of refined metal from the cold hearth apparatus. The refined metal passes as a stream from the cold finger orifice and is processed into a metal structure having desirable grain structure. A .preferred method for forming such a structure is by spray forming.
The above process described in the GE patents has the capability of operating continuously for an extended period of time and, accordingly, processing a large bulk of metal, if the rate of electroslag refining of metal and accordingly, the rate of delivery of the refined metal to the cold hearth approximate the rate at which molten metal is drained from the cold hearth through the cold finger orifice.
The apparatus utilized above, having an ingot as a consumable electrode, included a fixed relationship between individual parameters of the process and, in particular, between the intensity of the refined current, specific heat input and the melting rate. This fixed relationship entails undesirable interdependence between the rate of electroslag refining of the metal, the metal temperature and the rate at which the molten metal is drained from the cold hearth through the cold finger orifice. In addition, there are some problems concerning preparation of a large consumable electrode metal ingot.
For all of the above-mentioned reasons, there is a need for a new and improved electroslag refining apparatus and method to produce high quality, refined metal articles.
This need is satisfied by providing in the present invention a method for refining metal comprising the steps of: providing metal with nonspecification chemistry and microstructure; introducing the metal into an electroslag refining vessel containing molten slag in a top sleeve of the vessel, said top sleeve of the vessel being a non-consumable electrode with at least one pair of symmetrical leads; contacting the molten slag in the vessel with the metal; passing a sufficient amount of electric current through the slag for causing the metal to melt or overheat at surfaces where the metal contacts the slag; removing inclusions or impurities from the metal exposed to the slag; passing droplets of the metal formed from such melting or overheating through the slag; collecting the descending molten metal in a hearth positioned beneath the electroslag refining vessel. The inventive method further comprises the step of: rotating or stirring the slag with the metal in the top sleeve of the refining vessel with an electromagnetic force. The electric current being passed through the slag with at least one pair of symmetrical leads passes through a circuit comprising a power supply, the molten slag, and said refining vessel to cause resistance heating of the slag. The circuit can also include the liquid refined metal. The electroslag composition is a salt containing calcium fluoride. In yet another aspect of the invention, the method further comprises the step of: providing a cold finger bottom pour spout at a bottom of the hearth for permitting the liquid metal to pass through the spout as a metal stream. The rate at which molten metal is drained from the hearth is about equivalent to the rate at which metal is melted. Still yet, the invention comprises the step of: forming the metal stream into an article having specification chemistry and microstructure. The article can have a preform shape, be atomized into powder, cast into a rod, spun into ribbon, or used as a filler metal for cladding or surfacing.
The present invention in another of its broader aspects may be accomplished by an apparatus for producing refined metal comprising a metal refining vessel adapted to hold a metal refining molten slag, means for supplying refining current to the molten slag, means for introducing filler metal into the vessel in touching contact with the molten slag, electric supply means for supplying refining current to the top sleeve of the vessel as a non-consumable electrode and through the molten slag and the metal pool to the current lead in the bottom sleeve of the vessel and for keeping the refining slag molten, a hearth beneath the metal refining vessel, the hearth receiving and holding electroslag refined molten metal in contact with a solid skull of the refined metal in contact with the hearth, a middle sleeve, operatively positioned between the metal refining vessel and the hearth, electrically insulated therefrom and including a control level mechanism. The top sleeve of the vessel further may comprise a means for rotating the molten slag together with the molten metal. The hearth may have a cold finger orifice, operatively positioned below the hearth for receiving and dispensing as a stream, molten metal processed through the electroslag refining process and through the hearth.
Still another aspect of the invention the formation of relatively large metal ingots having a uniform composition and a desirable fine microstructure without utilizing the extensive multistep process of the prior art.
Another aspect of the present invention provides a molten stream of above specification metal from below specification metal from forms including ingots, bars, tubes, plates, rods, etc., and also including loose materials (powder, granules, shavings, pieces of irregularly shaped metal) and liquid metal.
A further aspect of the present invention provides an apparatus and methods for overcoming interdependence between the rate of electroslag refining metal, metal temperature and the rate at which molten metal is drained from the cold hearth through the cold finger orifice.
Still another aspect of the present invention is to provide apparatus and methods for actively stirring the metal and the slag.
Other advantages of the present invention will be apparent from the following description and the accompanying drawings.